In the printing industry, various processes are used to avoid and correct compass and register errors. Compass or register errors occur when an image is imprinted at an incorrect location on a printing image carrying element or on a printing medium. The term “in compass” or “in register” identifies a condition in which the printed image is imprinted at the correct location on a printing medium, i.e., the image is imprinted at the proper level or in the proper position. In multi-color printing, the term “compass” is standardly used, while in mono-color printing, the term “register” is used. To avoid compass and/or register errors, register marks or marks that are imprinted on a carrying element of the printing machine or on the printing medium, are often used in order to check the printing medium's register or compass; i.e., whether the compass or register is free of error. In the foregoing, the term “mark” will be used exclusively to describe both terms (compass and register).
The carrying element is often the conveyor belt that carries the printing medium or, in the case of electrophotographic printing, the master cylinder that carries the printed image. Marks come in various shapes, sizes, and colors. Outside of the printing machine, marks are manually measured by an operator with the aid of a magnifying lens and measuring mechanisms. Inside the printing machine marks are automatically measured with the aid of sensors, whereby any shifting of the printed image will be identified. Identification of a compass and/or register error is accomplished either before imprintation, as a means of calibrating the printing machine, or during the imprintation, i.e., on the fly.
The quality of the correct positioning of the compass and/or register is a significant factor for the quality of the printing result. With increasingly higher demands for quality printing and for proper positioning of the imprinted image, attempts are made to adjust the register with even greater precision.